The present invention relates to micromirror.
Oscillatory micromirrors in the form of an imaging concave mirror, are discussed in the publication xe2x80x9cMicroelectromechanical Focusing Mirrors,xe2x80x9d IEEE (MEMS 1998), Catalog No. 98CH36176, pages 460-465, of D. M. Burns and V. M. Bright, and have numerous technical applications in, for example, displays, scanners, and optical monitoring systems. They may be implemented using silicon technology on a silicon wafer; in the case of large mirror surfaces with areas of up to several mm2, such as may be needed for use in surveying lasers or in passenger car interior monitoring systems, the entire wafer thickness may be utilized as space for deflection of the mirror. The oscillatory excitation of the micromirror may be accomplished electrostatically. One difficulty with oscillatory micromirrors of this kind having large mirror surfaces may occur when large oscillation amplitudes (several tens of degrees), such as those that may be required for the aforementioned applications, are simultaneously present. In the case of electrostatic oscillatory excitation of the micromirror, which is achieved by way of electrodes correspondingly mounted beneath the mirror, voltages of up to several hundred volts are believed to be necessary in order to attain such oscillation amplitudes (see Petersen, IBM J. Res. Develop. 24 (1980) 631; Jaecklin et al., Proc. IEEE MEMS Workshop, Fla., USA (1993) 124). The use thereof in the automotive sector, and the generation and management thereof on a micromechanical component, may be problematic. In addition, if what is desired is to operate the oscillatory micromirror in resonant fashion, this may require a complex electronic analysis system that detects very small changes in capacitance between the mirror surface and the excitation electrode located therebeneath.
It is therefore an object of an exemplary embodiment of the present invention is to provide an oscillatory micromirror that can be excited to large oscillation amplitudes with low voltages even when the mirror surface is large, thus making possible utilization even in economical applications.
The micromirror according to an exemplary embodiment of the present invention having the characterizing features of the the advantage that a small flexural oscillation amplitude is sufficient to generate a large amplitude for the torsional oscillation, induced by the flexural oscillation, of the cantilevered mirror surface about the torsion axis defined by the torsion beam. Especially if the frequency of a flexural oscillation is identical to a resonant frequency of a torsional oscillation, the amplitude of that torsional oscillation is particularly large. Because of the low amplitude of the flexural oscillation, however, the vertical movement of the mirror brought about thereby is still very small, so that, for example, any defocusing of a reflected light beam is believed to be negligible.
The mirror surface can assume almost any geometric shape, and for example can be a planar disk, a planar rectangle, or a planar square, or can have the shape of an imaging concave mirror with a circular base outline. Its size can range from a side length of a few xcexcm up to several mm.
It is also believed to be advantageous if two oppositely located torsion beams are mounted on the oscillatory mirror surfacexe2x80x94which, for example, has the shape of a squarexe2x80x94in such a way that the torsion axis defined thereby does not coincide with an axis of symmetry of the mirror surface. It is also believed to be advantageous if, in the case of multiple torsion axes, at least one of the torsion axes formed by the respective number of torsion beams is not an axis of symmetry of the mirror surface.
In order to avoid excessive amplitudes of the flexural oscillation exciting the torsional oscillation, it is also believed to advantageous if the frequency of the flexural oscillation is set such that the resonant frequency of the flexural oscillation is very different from the resonant frequency of the torsional oscillation, so that a resonant torsional oscillation does not also simultaneously result in a resonant flexural oscillation. The resonant frequency of the flexural oscillation(s) and/or the torsional oscillation(s) can be set very easily, and independently of one another, by way of the geometry of the flexural beam and the torsion beam, their mechanical properties, and their composition.
A torsional oscillation of the mirror surface about a torsion axis defined by a torsion beam can be excited via a flexural oscillation of a flexural beam if the torsion beam and the flexural beam joined thereto do not lie on one line or axis, and thus enclose an acute or obtuse angle. In particular, the torque on the mirror surface about the torsion axis brought about by the flexural oscillation is believed to be particularly great if the angle between torsion beam and flexural beam is 90xc2x0.
The oscillatory micromirror according to an exemplary embodiment of the present invention can furthermore be manufactured very advantageously and easily if the mirror surface, the torsion beams, and the flexural beams are patterned out of a silicon wafer, since in this case it is possible to utilize available silicon-based etching techniques and surface micromechanical patterning methods.
When the torsional oscillation of the oscillatory micromirror is generated according to exemplary embodiment of the present invention, it is also believed to be advantageous, especially in combination with patterning out of a silicon wafer, that an additionally necessary electrical triggering and interconnect system for the oscillatory micromirror can also be accommodated on the wafer.
The excitation via a flexural oscillation necessary in order to generate a torsional oscillation can be accomplished in very advantageous fashion by the fact that at least one thermoelectric or piezoelectric flexural transducer, which is joined to the flexural beam and induces flexural oscillations in the flexural beam, is mounted on the flexural beam at least on one side. This can advantageously be achieved by the fact that a flexural beam made of silicon is surface-doped and is thus usable as a thermoelectric flexural transducer.
In order to allow the resonant frequency of a torsional oscillation of the mirror surface to be adjusted as easily as possible, it is also believed to be advantageous if there is applied on at least one torsion beam a piezoresistive or piezoelectric transducer, in particular in the form of a thin-film transducer, so that its electrical properties or its measurement signal (piezo voltage) changes as a function of the amplitude and frequency of the torsional oscillation. The electrical signal of this transducer on the torsion beam can thus be coupled into the feedback circuit of a resonator which serves to excite the flexural oscillation via the thermomechanical or piezoelectric flexural transducer mounted on the flexural beam. The measured signal of the piezoresistive or piezoelectric transducer that detects the amplitude of the torsional oscillation thus very advantageously regulates the frequency of the flexural oscillation.
To simplify the electrical contacting of the piezoresistive or piezoelectric transducers mounted on the torsion beams and/or to create an electrical conductive connection between these transducers on the torsion beams, and to create an electrical connection between the thermoelectric or piezoelectric flexural transducers mounted on the flexural beams in order to generate the flexural oscillation, it is also believed to be very advantageous if the mirror surface and/or the torsion beams and/or the flexural beams are at least locally metallized on the surface.
In addition, it is very advantageous that the micromirror according to an exemplary embodiment of the present invention, in order to generate the torsional oscillation, requires only low voltages and, in particular, no counterelectrode, for example as in the case of electrostatic excitation. The manufacturing process can moreover be performed using available methods, and in particular a fully CMOS-compatible manufacturing process is also possible.